1. Field of the Invention
The invention relates generally to capacitive moisture sensors. More particularly, embodiments of the invention provide sensing and measurement devices for monitoring mist propagation conditions through the use of electric fields.
2. Description of Related Art
The general concepts surrounding fringing sensor devices have been well documented and have even been discussed in a few patents on fringing sensor devices. For example, U.S. Pat. No. 2,219,497, entitled “Electrostatic Type Test Electrode,” shows several embodiments of electrodes which use fringing fields as a sensing technique, which patent is incorporated by reference herein in its entirety. This patent describes devices that are more conductive than capacitive, which is also a characteristic of devices described in U.S. Pat. No. 6,239,601, entitled “Thickness Measurement Device for Ice, or Ice Mixed with Water or Other Liquid,” as well as in U.S. Pat. No. 4,639,831, and Great Britain Patent No. 2149922 entitled “Capacitive Moisture Sensor and Process for Producing Same.”
Other sensors, moisture monitoring systems, and electrical circuits instructive as to embodiments of systems, devices, and methods of the invention include for example any of U.S. Pat. Nos. 2,043,241, 2,076,441, 2,121,103, 2,207,906, 2,219,497, 2,285,152, 2,357,984, 2,373,846, 2,570,218, 2,455,450, 2,508,081, 2,570,218, 2,611,021, 2,662,408, 2,729,786, 2,817,234, 2,849,882, 2,969,186, 3,046,479, 3,079,089, 3,112,647, 3,129,415, 3,177,427, 3,218,551, 3,231,815, 3,278,843, 3,443,213, 3,515,987, 3,518,535, 3,519,940, 3,622,873, 3,628,132, 3,646,434, 3,694,742, 3,778,705, 3,781,672, 3,826,979, 3,862,571, 3,986,110, 3,991,939, 4,187,460, 4,190,201, 4,194,691, 4,228,393, 4,272,718, 4,332,105, 4,502,288, 4,517,547, 4,545,396, 4,626,774, 4,639,831, 4,657,039, 4,675,596, 4,683,904, 4,795,965, 4,805,070, 4,806,848, 4,845,421, 4,858,063, 5,159,247, 5,210,500, 5,224,769, 5,225,783, 5,245,295, 5,301,542, 5,311,140, 5,313,168, 5,398,547, 5,414,368, 5,479,104, 5,491,423, 5,537,109, 5,751,071, 5,780,718, 5,783,743, 5,844,138, 5,870,302, 6,249,130, 6,373,263, 6,552,552, 6,575,621, 6,628,501, 6,647,782, 6,756,793, 6,911,829, 7,049,829, 7,129,713, 7,170,302, 7,176,700, 7,183,779, 7,267,002, 7,471,093, 7,834,646, 7,971,482, and 8,009,053, as well as U.S. Published Patent Application No. 2001/038,440, which patents and publication are each incorporated by reference herein in their entireties.
It has been found by experimentation that fringing capacitors with the sensor's metal runs in direct contact with the cover plate of this sort are 73 times more conductive than they are capacitive. Moisture ions pass directly through the cover plate to the runs and short them out making them insensitive to capacitive changes due to the moisture found on top of the glass. An air gap, as described in this invention, acts to insulate the runs from the moisture and prevent them from shorting out. Note that the capacitive change in a sensor such as this is approximately 60 pico-farads from dry to fully wet and this is swamped out by the conductivity which can be as small as 228 mega-ohms but which is 73 times more conductive than capacitive.